1. Field of the Invention
One embodiment of the present invention relates to a method for manufacturing an SOI substrate.
2. Description of the Related Art
In recent years, an integrated circuit using an SOI (silicon on insulator) substrate in which a thin single crystal semiconductor layer exists on an insulating surface, instead of a bulk silicon wafer has been developed. The use of an SOI substrate leads to a reduction in the parasitic capacitance between a drain of a transistor and the substrate; thus, SOI substrates have attracted attention for their ability to improve performance of semiconductor integrated circuits.
One of known methods for manufacturing an SOI substrate is a hydrogen ion implantation separation method (for example, see Patent Document 1). A method for manufacturing an SOI substrate according to a hydrogen ion implantation separation method is summarized below. First, hydrogen ions are implanted into one silicon wafer by an ion implantation method to form a microbubble layer in the region at a certain depth from a top surface of the silicon wafer. Next, the silicon wafer is bonded to the other silicon wafer with a silicon oxide film provided therebetween. Then, heat treatment is performed thereon, so that a part of the one silicon wafer is separated in a thin film shape along the microbubble layer serving as a cleavage plane. Accordingly, a single crystal silicon film can be formed over the other silicon wafer.
Further, a method for forming a single crystal silicon layer over a base substrate made of glass according to such a hydrogen ion implantation separation method has been proposed (for example, see Patent Document 2). Glass substrates are more suitable for an increase in area and are less expensive than silicon wafers; thus, an SOI substrate which has a large area and is inexpensive can be manufactured by using the glass substrate as a base substrate.
Further, in Patent Document 2, it is proposed that a silicon nitride film is provided between a base substrate and a single crystal silicon layer so as to prevent impurities in the base substrate or the like from diffusing into the single crystal silicon layer.
A single crystal semiconductor substrate is irradiated with accelerated hydrogen ions, whereby the hydrogen ions are implanted or doped into the single crystal semiconductor substrate at a certain depth from a top surface thereof, so that an embrittled region is formed. Then, heat treatment is performed thereon, whereby hydrogen contained in the embrittled region is vaporized and expanded, so that a cleavage plane is formed in the embrittled region.